The invention relates to a spark plug for the internal combustion engine of a motor vehicle, of essentially elongated general shape, comprising:                two coaxial electrodes: an inner electrode with an axis called a central electrode and an outer electrode called a body surrounding the central electrode; and        an electrically insulating block called an insulator, placed between the central electrode and the body, of annular shape and having an annular shoulder.        
Plasma-generating plugs are high frequency multi-spark ignition systems, capable of providing the best quality ignition conditions in controlled-ignition engines, while at the same time reducing polluting emissions, in particular with a lean mixture. On the other hand, they are subject to fouling, in particular when cold.
Like all plugs, they are classified in terms of a heat range. This heat range takes into account their thermal behavior at particular moments of engine operation. It expresses in particular their ability to withstand temperatures which are high enough to remove the fouling by pyrolysis, without causing any ‘pre-ignition’.
The publications FR2,859,830, FR2,859,869 and FR2,859,831 mention a multi-spark plug, called a cold plug because its temperature does not rise fast enough to prevent fouling. On such plugs, in fact, the accumulation has been observed of a deposit of carbon on the insulator, which significantly reduces the insulation which is required between the tip of the central electrode and the body. With poor insulation, the high voltage supply of the plug could then be too low to be able to cause the required spark-generating ‘flashovers’.
To prevent the formation of deposits containing carbon, in particular when cold, on the insulator of the plug exposed to the environment in the combustion chamber, an effort can be made to increase the temperature of the insulator, so as to promote the destruction of the deposits by the phenomenon of pyrolysis, of which the efficiency depends on the thermal resistance of the plug assembly, including that of the insulator.
The measures usually taken to increase the temperature of the insulator are limited by the occurrence of ‘pre-ignition’ on the plugs, when they reach operating temperatures which are too high.